create "internal use" command verbs: _dump, _help

- based on abstract class `Cask::CLI::InternalUseBase`
- internal-use class file names follow `internal_<verb>.rb`, but the "internal" is dropped for the CLI
- internal-use verbs start with underscore at CLI
- internal-use verbs are invisible in the regular usage document
- This PR adds `_dump` command
- adds `_help` command
- includes refactoring of `cli.rb` and `cli/*.rb`
- this is intended as a playground for new functionality *eg* `_appcast`
- no tests on internal verbs: they are always considered unspecified and unstable